1. Field of the Invention
The present invention relates to a phase-change optical recording medium (hereinafter may be referred to as “optical recording medium”) and recording and reproducing method of the phase-change optical recording medium in which crystallization in recorded mark is suppressed.
2. Description of the Related Art
In late years, there has been an increase in development of phase-change optical recording media in which phase-change materials are used as protective layers. The phase-change optical recording media which are in practical use are mainly the phase-change optical discs.
In a phase-change optical disc in general, a specified groove is formed on a transparent plastic substrate and a thin film is formed on the substrate. The plastic material used for substrate is mainly a polycarbonate and injection molding is often used for groove formation. The thin film formed on the substrate is a multilayer film and it is basically composed of first protective layer, recording layer, second protective layer and reflective layer in this sequence from the substrate.
For the first and second protective layers, oxides, nitrides and sulfides are generally used and among them, ZnS—SiO2, a mixture of ZnS and SiO2 is commonly used.
For the recording layer, a phase-change material containing SbTe as main element is often used. In particular, examples include Ge—Sb—Te, In—Sb—Te, Ag—In—Sb—Te, Ge—In—Sb—Te and Ge—Sn—Sb—Te and other than these, Ge—Te, In—Sb, Ga—Sb and Ge—Sb are also used.
Metal materials are used for the reflective layer and metal materials such as Al, Ag, Au and Cu and alloys thereof are preferably used for their appropriate optical properties and heat conductivity. In addition, different layers are formed or a multilayer is formed for each layer to have so-called “insert layer” or “interface layer” between above-mentioned each layer for the purpose of improving various disc properties.
Various film-forming techniques such as resistance line heating, electrical beam vapor deposition, sputtering, CVD, and the like may be used for the formation of these multilayer and among them, sputtering is often used for its excellent productivity. A resin layer is coated by spin coating after formation of these layers for the protection of thin film.
In the resultant phase-change optical discs, the phase-change material used for the recording layer is in an amorphous condition and it is generally initialized for crystallization.
The initialization of above-produced phase-change optical discs is usually conducted by irradiating a laser beam from a semiconductor laser having a width of several micrometers and a length of several dozen to several hundred micrometers while rotating the disc and moving the laser beam in radius direction. The laser beam irradiation is often equipped with a focusing function for more effective irradiation.
In the phase-change optical disc produced above, it is possible to form a desired amorphous mark (hereinafter may be referred to as “recorded mark”) by irradiating an optionally determined laser emission pattern (hereinafter may be referred to as “recording strategy”). Moreover, a direct overwrite (DOW), in which erasing and recording are performed simultaneously, is possible in the phase-change optical disc. In “erasing”, amorphous mark is recrystallized and in “recording”, an amorphous mark is formed from crystal condition.
One of often used recording strategies is a three-value control (Pw>Pe>Pb), which include recording power (Pw), erasing power (Pe) and bias power (Pb). A specified mark length is recorded by combining these and various pulse width. The mark length control is extremely important because EFM modulation used in CD or EFM+ modulation used in DVD as a modulation method for data recording and reproducing is a mark edge recording method. In general, jitter property is used for evaluation of the mark length control.
Such phase-change optical discs are applied to CD-RW, DVD+RW, DVD-RW, DVD-RAM, HD-DVD, Blue-Ray Disc, etc. and are widely spread for audio visual application and information recording application of computers.
In recent years, improvement of recording speed of these optical discs is hoped owning to larger amount of digital capacity. For high-speed recording of the phase-change optical discs, rewrite functions at higher recording linear velocity in a wider range of recording linear velocity are required. The higher recording linear velocity corresponds to the maximum recording linear velocity and a wider range of recording linear velocity corresponds to a recordable range of linear velocity. These will be explained below.
When two recording methods, CAV recording in which a recording is performed at a constant recording rotational frequency and CLV recording in which a recording is performed at a constant linear velocity are considered, CAV recording is more preferable. In CLV recording, a rotational frequency changes depending on a radius value and higher rotational frequency is required near inner periphery.
Therefore, the maximum linear velocity at the innermost periphery is determined by the rolling capacity limit of a recording/reproducing apparatus for optical disc. When CAV recording is employed, it becomes possible to perform a recording at a linear velocity higher than above limit by performing a recording faster at outer periphery than at innermost periphery.
For example, if the rotational frequency limit of a recording/reproducing apparatus for optical disc is 10,000 rpm, linear velocity is approximately 25 m/s at 24 mm radius and this corresponds to approximately 7 double speeds when standardized with DVD standard linear velocity of 3.5 m/s. When recording at a linear velocity higher than this, it is necessary to perform CAV recording in a determined radius range or in the entire surface of a disc. Or, it is necessary to perform ZCLV recording in which multiple recording linear velocities corresponding to radius value of a disc are employed.
Since data transfer speed itself commensurate with recording linear velocity, it is necessary to perform CAV recording for recording at a linear velocity higher than the rotational frequency limit of a recording/reproducing apparatus for optical disc. Therefore, rewrite function at a determined range of recording linear velocity is also required as well as improving the maximum linear velocity for obtaining better recording linear velocity.
In addition, when CAV recording is performed in CD or DVD, since disc size is 120 mm in diameter, the ratio of recording linear velocity at outermost periphery to recording linear velocity at innermost periphery (recording linear velocity at outermost periphery/recording linear velocity at innermost periphery) calculated is approximately 2.4. For a 4 double-speed disc of DVD+RW, the range of recording linear velocity required for CAV recording is 5.8 m/s to 14.0 m/s. “4 double-speed” in here means the linear velocity is 4 times the standard linear velocity of DVD, 3.5 m/s.
On the other hand, as the maximum recording linear velocity increases, the range of recording linear velocity also increases by necessity. As mentioned above, it turns out that the recording linear velocity ranges from 5.8 m/s to 14.0 m/s (range width of 8.2 m/s) for 4 double-speed discs while the recording linear velocity ranges from 11.5 m/s to 28.0 m/s (range width of 16.5 m/s) for 8 double-speed discs. Therefore, it is necessary to widen the range of recording linear velocity as well as to increase the maximum recording linear velocity for improving the recording linear velocity of CAV or ZCLV recording.
It has been discussed that the need for CAV or ZCLV recording comes from the rotational frequency limit of an optical disc in a recording/reproducing apparatus for optical disc. The CAV recording is also a significant technique for speeding up of a random access recording because rotational frequency is constant in CAV recording and there is no need to adjust the rotational frequency depending on a radius value for recording. Speeding up of a random access recording equals to speeding up of a data transfer and it also leads to the improvement of recording speed actually felt by users consequently.
The related arts regarding the high-speed recording and CAV recording described above can be found in Japanese Patent Application Laid-Open (JP-A) Nos. 2004-203011, 2004-164850, 2004-164849, 04-286683, 06-103609 and Japanese Patent (JP-B) No. 3474714, for example.
For example, an optical recording medium using phase-change materials based on InSb (Technical Digest ISOM '04 p.266 “In—Sb Phase-Change Material for 16× DVD-Rewritable Media”), SnSb (JP-A No. 2004-203011), and the like is proposed as phase-change recording materials capable of high-speed recording. However, both proposals only refer to high-speed recording and there is no description for the range of recording linear velocity required for CAV recording. In addition, with reference to the optical recording medium using phase-change materials based on GaSb, GeSb (JP-A No. 2004-224040, No. 2004-224041 and No. 2004-322630), BiGeTe (JP-A No. 2004-259443 and No. 2004-255889), and the like, high-speed recording and CAV recording are described.